In this section recent achievements in the designing and fabrication of mode field adaptors and power combiners are described. Configurations include pump, pump and signal, and only signal coupling; they are characterized by the signal and/or pump radiation coupling efficiency. These components use different types of fibers that are fabricated in various configurations and in different fabrication processes.
5.1. Pump Power Combiners
The pump power combiner in N × 1 configuration is used when N pump sources (typically with an MM fiber) have to be combined into one output fiber [17
]. The lack of a feed-through signal fiber simplifies the design and fabrication process of this component. The most recent achievements are presented in Table 1
. The 7 × 1 configuration seems to be the most common approach, but lower or higher numbers of pump ports can also be found.
An interesting approach for improving pump power transmission efficiency using a capillary tube with refractive index matching the refractive index of the clad of the MM fiber was presented in [51
]. The capillary tube acts as a clad of MM fibers and helps to keep the radiation inside the fiber bundle. This allows for the achieving of higher pump transmission efficiency compared to the combiners in which the refractive index of the capillary tube matches the refractive index of the MM fiber core. In this second case, the radiation leaks out from the fiber into the capillary causing additional transmission losses.
Another critical parameter in power combiner fabrication is the ratio between the diameters of the input fiber bundle and the output fiber. When input and output fibers with large core and clad diameters are used a small taper ratio (TR) is needed, which makes it easier to preserve the brightness of the pump light. However, it is often required to use larger TR. For example, when an output port is a DC fiber with a clad diameter of only 125 µm and seven MM 105/125 µm fibers are used at the input, then the required TR is 3. Presented in our previous work [53
] power combiner requiring TR twice as large as presented power combiner in [51
], had BR below 1, which indicates losses resulting from the tapering of the input fiber bundle from the initial 375 µm diameter to the diameter of the output DC fiber (125 µm). Using input and output fibers with much larger clad diameters [51
] helps to maintain high beam brightness and achieve handling of a very large amount of pump power. In a paper [55
] two configurations of power combiners are presented: 3 × 1 and 7 × 1 capable to handle approximately 2.1 kW and 4.72 kW, respectively.
It is challenging to design and fabricate a power combiner with relatively small cladding diameter of the output fiber, and it is also challenging to combine more than ten input fibers in the bundle. In a paper [56
] a 16 × 1 power combiner was fabricated by double bundling of the input fibers. Scheme of the fabrication process is presented in Figure 5
a. A 32 × 1 power combiner has been demonstrated in [57
]. In this case, the input fiber bundle not only has 32 pump ports, but also an additional large central fiber which is used to guide the Amplified Spontaneous Emission (ASE) light and unwanted back reflections from other parts of the setup.
An interesting design described in [52
] uses a 7 × 1 power combiner with a side-pumping approach. Comparing to the other power combiners presented in this subsection, its fabrication process is quite sophisticated. The DC fiber is placed and fused in the capillary tube whose outer diameter decreases in the output direction (this was achieved by hydrofluoric acid bath). Subsequently, MM fibers are spliced to the front side of the capillary tube (with larger diameter, as shown in Figure 5
b). In this configuration the capillary tube with gradually decreasing outer diameter plays the role of additional cladding layer for the DC fiber.
5.2. Pump and Signal Power Combiners
The design and fabrication process of pump and signal combiners is far more challenging than in the case of pump power combiners, because, besides pump power being transmitted from tapered MM fibers into the inner cladding of the output DC fiber, there is also a signal fiber, which tapering decreases its initially small core diameter. This may result in its mode field mismatch with the output fiber, and thus significant signal transmission losses. Many scientific reports presenting theoretical modeling and fabrication of those kind of components can be found in the literature [58
]. A (6 + 1) × 1 configuration is the most common configuration of pump and signal power combiner. However, as in the case of pump power combiners, in this type of power combiner different configurations with a lower or higher number of pump ports can be found in the literature. The dimensions and number of input fibers must be arranged in a way that the signal feed-through fiber is placed in the center surrounded by pump fibers. Some recently presented fabricated pump and signal power combiners are shown in Table 2
One of the approaches to preserve high signal transmission is fabrication of the power combiner in a side-pumping scheme. An example of this type of combiner in (4 + 1) × 1 configuration is presented in a paper [64
]. Here, each pump fiber was spliced to an intermediate coreless fiber, which was then fused around the passive DC fiber (Figure 6
a). In this approach, minimal impact from the fabrication process on the signal fiber is guaranteed.
More often than not power combiners are fabricated in an end-pumping scheme by a TFB technology. As it can be seen in Table 2
, power combiners with output DC fibers diameter of below 200 µm are not very common. This is because they require high TR, which makes it very difficult to preserve high pump power and signal coupling efficiency due to the loss in beam brightness and MFD mismatch of signal input fiber with output fibers. In our previous work [65
], a power combiner in an (5 + 1) × 1 configuration is presented with an output SM DC passive fiber with inner cladding diameter of only 125 µm. To improve signal transmission efficiency we have used five pump fibers (instead of six) and signal feed-through fiber with lower clad diameter of 80 µm than commonly used 125 µm (Figure 6
b). This allowed us to decrease the TR and achieve signal transmission on the level of 94.5%. In a paper [71
] a (7 + 1) × 1 configuration is reported with 133 µm diameter of the output DC fiber, and with 160 and 125 µm clad diameter of the input signal and pump fibers, respectively. It is an example of a power combiner with DC photonics crystal fibers (PCFs). Here eight input fibers were only fused together to form a bundle. As an output fiber an SM PM DC fiber with MFD = 15 µm with an air-cladding was chosen. In order to match the input fiber bundle with the output fiber an intermediate PCF fiber was used, on which tapering was performed.
(6 + 1) × 1 power combiners utilizing single mode input signal fibers, 105/125 µm pump fibers and a 25/250 µm DC fiber at the output are presented in papers [16
]. In paper [66
] a conventional SMF28e fiber is used as a feed-through fiber and the signal transmission efficiency was improved from 51% to 94% thanks to the TEC technique. In a paper [16
], the authors reported a signal and pump power transmission efficiency of 94%. In this case, the custom-made fixture was used for power combiner fabrication, and the authors pointed out that the fabrication process may include etching of fibers, thus decreasing their clad diameter without any impact on their core.
Another interesting approach to achieve high signal transmission efficiency is presented in paper [67
]. With vanishing core technology the authors developed a polarization maintaining (6 + 1) × 1 power combiner with a signal and pump power transmission efficiency of 91% and 98%, respectively. In this method before the tapering process the initial mode field is determined by the refractive index difference of n1
(of the “vanishing” core) and n2
(of the secondary core) as it was explained by the authors [67
] (Figure 7
a). After tapering, the central core is too small to guide the light, thus we may assume that it vanished. Therefore, at the taper output the mode field is determined by the refractive index difference of the second and third layers (n2
, respectively). In order to use this method a special signal fiber with an additional layer of the secondary core must be used.
High signal and pump transmission of more than 97% and 98%, respectively, was achieved in a paper [68
]; the same signal fiber was used in the input fiber bundle and at the output of the (6 + 1) × 1 combiner. Such high transmission efficiency was possible due to the reduced input signal fiber cladding by the chemical corrosion and two-step splicing method. The fabrication process differs from that previously presented. At first six pump fibers were placed in the capillary tube and contacted with its wall forming a ring shape. Then, the bundle was tapered down creating an approximate hexagon hole in the middle with a diameter of approximately 100 µm. A scheme of the cross sectional area of the bundle is presented in Figure 7
b (left image). The clad of the input signal fiber was reduced to less than 100 µm by chemical corrosion. Then the signal fiber was inserted into the hole of previously prepared bundle. Two-step splicing meant that, at first, the signal fiber from the bundle was spliced with the output fiber (Figure 7
b, upper right image) and then the rest of the bundle—pump fibers were moved to the output fiber and carefully fused and spliced (Figure 7
b, bottom right image).
In a paper [69
] a (6 + 1) × 1 power combiner was designed in a way that the input fiber bundle diameter is only slightly larger than the output fiber diameter. Authors have used home-made signal fibers at the input and output with core/clad diameter of 30/220 µm and 30/600 µm, respectively. In their design, signal fibers have the same core diameter, and the combiner fabrication only requires fusing of the input fiber bundle, without the tapering, resulting in a small change of the input signal fiber core and very high signal transmission efficiency.
Another unconventional (8 + 1) × 1 configuration was presented in [72
]. Just like in the case of power combiners with an output DC fiber diameter of 400 µm and above, very high signal transmission efficiency was achieved; however, the signal and output fibers were characterized with a very large core diameter of 100 µm. The fabrication process developed by the authors is slightly similar as in a paper [68
]. Here, eight pump fibers are arranged in a ring configuration by holding clamps and a pretaper process is performed on them, reducing the diameter of each MM fiber from 125 to 100 µm. Then, the input signal fiber is inserted into the middle of the pretapered bundle, pump fibers are slightly twisted around the signal fiber for better attachment, and then the whole structure is fused, cleaved, and spliced to the output fiber. As it was described in this case, as well as in the papers [16
], the input signal fiber is not tapered down, but it is inserted into the properly prepared input fiber bundle and/or its decreased clad diameter is achieved by chemical etching, resulting in very high signal transmission efficiency.
Another interesting approach applies a built-in mode field adaptor [14
]. Such a solution was chosen by authors of a work [70
] however, an intermediate fiber was not used. Here the input fiber bundle and the output DC fiber were both tapered down in order to match their mode fields. Then they were spliced, resulting in a transmission efficiency above 87% and 98% of the signal and pump power, respectively.
5.3. Signal Power Combiners
Currently the output power of fiber lasers have reached the kilowatt (kW) level however, phenomena such as thermal damage, nonlinear effects and modal instabilities limit the power level at the output of a single fiber laser. An interesting and relatively simple solution uses a beam combining technique by merging the output power of several hundred watts (from several lasers) into one large core delivery fiber [26
]. Incoherent beam-combining based on fiber technology is particularly interesting because it allows preserving all the advantages of the all-fiber laser construction technique. Recently presented signal power combiner are shown in Table 3
LMA type fibers are typically used as input fibers in signal power combiners. As an example of such configuration a 7 × 1 signal power combiner capable of handling a 2.5 kW signal power with M2
= 6.5 was presented in paper [80
]. Another7 × 1 power combiner presented in [82
], shows the development and transmission efficiency scaling of combiners presented by the same research group in papers [76
]. In a combiner reported in [84
], the output fiber had a 100 µm core diameter, resulting in signal transmission on the level of ~99%, M2
≈ 10 and measured power handling of about 500 W. In a paper [76
] the output fiber core diameter was 2-fold smaller (50 µm), and the signal transmission efficiency was at the same level as previously however, the M2
factor was improved (M2
= 6.0/6.3). In all the above-presented and discussed signal power combiners by this research group the same signal fibers were used as inputs: 20/130 µm (NA = 0.08/0.46) [76
]. With smaller output fiber core diameter they were able to achieve ~99% signal transmission efficiency with M2
= 6.0/6.3 [76
], and later [82
] 98% signal transmission efficiency with improved M2
= 4.3 and measured power handling of 6.26 kW. In these papers a fluorine-doped low-index capillary was used, forming with input fibers a structure with seven cores and a cladding created by low-index capillary tube. In a recent report [83
] a 7 × 1 power combiner achieved power handling of above 10 kW with measured M2
= 5.37 (at 14 kW of the output power). In this case the authors used input fibers with much larger cladding diameter (400 µm); this diameter can be decreased by chemical etching. The signal power combiner fabricated in 3 × 1 configuration presented in [81
] was used to incoherently combine a supercontinuum source with power above 200 W. Because photonic crystal fibers have unique features which classical fibers do not have (e.g., high birefringence, larger single-mode areas, extremely low/high nonlinearity, and, in the case of hollow-core PCF, high damage threshold), therefore it is profitable to use PCF in components operating at high-power kW regimes; some theoretical and experimental achievements in this field have already been reported [85
5.4. Mode Field Adaptors
Mode field adaptors are not that complicated in fabrication as power combiners however, they are equally important in all-fiber MOPA configurations. A MFA may even be considered as a splice between two fibers with slightly different mode field diameters. Typically, they are fabricated just like power combiners by tapering or TEC method or the combination of both [88
] however, there are other solutions like mode field adaptors based on multimode interference in graded index multimode fiber (GIMF) [92
In most cases presented in Table 4
the authors have used, not only the TEC method, but they have combined it with the tapering process. Using each method separately will not give a mode field match as precise as they can achieve when combined together [91
]. This is why the authors of a paper [91
] were able to achieve approximately 95% signal transmission efficiency, despite a very large core and mode field diameter difference of used fibers. Two configurations of MFA, where at the input the authors have used firstly a 4/125 µm SM fiber, and secondly a 5/125 µm SM fiber, while a 15/130 µm LMA fiber was used as the output (in both cases) [94
]. The MFAs achieved signal transmission efficiency of approximately 93% and 91%, respectively, with only the TEC method used for fabrication process. The same research group presented two MFAs with the same input fibers [95
]; however, as an output fiber they have used a 25/250 µm LMA fiber with a larger MFD than in the previous case. Here the LMA fiber has MFD = 21.5 µm, while in the previous work the LMA fiber had MFD = 13.6 µm. This means much larger mode field mismatch with the input fiber, thus authors have used here TEC method combined with the tapering, which allowed to achieve signal transmission efficiency on level of >90%. In our most recent work, reported recently [96
], two types of MFAs are mentioned. The first one connects two types of double clad LMA fibers, thus it guide both the signal and pump power with an efficiency of 92%. In the case of this MFA we have used only tapering in the fabrication process; however, in the case of the second MFA, we combined tapering with the TEC method. It was necessary to use both methods because of the large mode field mismatch of used fibers.